This invention relates to a semiconductor laser device and a method for manufacturing the same. More particularly, this invention relates to a semiconductor laser device that is adapted to operate on self-excited oscillation in a multi mode, instead of a longitudinal mode, at low return-light noise and reduced astigmatism, as well as a method for manufacturing the same.
There is known so-called a double-hetero semiconductor laser device having an active layer sandwiched between a first conductivity type cladding layer and a second conductivity type cladding layer. In a double-hetero device structure having a current blocking layer formed of a material with a narrow forbidden band gap, if the distance between the current blocking layer and the active layer is small, the device behaves as a refractive-index guiding structure to cause oscillation in a single longitudinal mode, thereby enhancing coherence.
On the other hand, if the distance between the current blocking layer and the active layer is so wide as not to absorb a portion of the light created by the active layer, the device behaves as a gain guiding structure giving no lateral difference in refractive index, making possible reduction of return-light noise. In such a structure, however, there is increase of astigmatism in unstable lateral mode, thus resulting in causing phenomenon of kink. Under such situations, efforts have being made for obtaining a semiconductor laser device that has an over-saturation absorbing layer formed in the cladding layer in a manner separated from the current blocking layer, in order to optimize the distance thereto from the active layer. In such a structure, characteristics of both the above-mentioned structures are satisfied, thereby reducing return-light noise as well as astigmatism. Semiconductor laser devices having a structure like this have been disclosed, e.g., by Japanese Provisional Patent Publications (Kokai) Nos. H7-86676 and H6-196810, that are adapted to cause self-excited oscillation in a multi mode instead of a longitudinal mode.
The semiconductor laser device of this kind is constructed as shown for example in FIG. 4 and FIG. 5. In the figures, reference character 21 denotes an n-type GaAs substrate, 22 (22a, 22b) is a first conductivity (n) type AlGaAs cladding layer, 23 is a non-doped Al.sub.a Ga.sub.1-a As (0.ltoreq.a .ltoreq.0.3) active layer, 24 (24a, 24b, 24c) is a second conductivity (p) type AlGaAs cladding layer, and 26 is an n-type GaAs current blocking layer, 27 is a Al.sub.b Ga.sub.1-b As (0.ltoreq.b.ltoreq.0.3) over-saturation absorbing layer. Over the second conductivity type cladding layer 24, a cap layer and a contact layer, both not shown, are provided. Top and bottom electrodes are respectively provided on top and bottom surfaces of the substrate having respective layers thus formed thereon, which are not shown. In this structure, the layers are formed such that the distance d between the over-saturation absorbing layer 27 and the active layer 23 is set to an optimum value so as to provide self-excited oscillation to bring the longitudinal mode into a multi mode. This realizes a low-noise characteristic.
In this structure, however, the over-saturation absorbing layer is provided between the current absorbing layer and the active layer. Consequently, where the distance d between the active layer and the over-saturation absorbing layer increases, the distance between the active layer and the current blocking layer also becomes distant. However, if the distance between the current blocking layer and the active layer is far, an electric current once narrowed by the current blocking layer spreads before reaching the active layer, being scattered to a certain extent in the cladding layer. As a result, the density of the current to the active layer decreases at its light emitting portion, raising a problem that operating current requirement and hence a consumption power are increased.
In the meanwhile, if the current blocking layer and the over-saturation absorbing layer are placed on opposite sides with respect to the active layer, the location of the current blocking layer is not affected by the restriction due to the over-saturation absorbing layer. However, where the current blocking layer and the over-saturation absorbing layer are placed on the opposite side in this manner, the light created is attracted by the over-saturation absorbing layer, and the light is distant from the current blocking layer. However, if no light comes to the current blocking layer, the difference in refractive index between the cladding layer and the current blocking layer decreases, and there arises a problem that the light confining effect is lessened with increased astigmatism. In order to reduce the astigmatism for the laser device, there is necessity that the current blocking layer and the over-saturation absorbing layer have to be arranged on the same side relative to the active layer.